1. Field of the Invention
This invention relates to precision control and more particularly to control of a fine stage positioned by two opposing E/I core actuators.
2. Description of the Prior Art
Fine stages are well known and are typically used, for instance in the semiconductor field, for moving reticles (masks) or wafers. For instance Trumper U.S. Pat. No. 5,196,745 discloses a device for controlling the relative position between a moveable platen and a stator having a range of movement in the 200 to 300 mm range. This is typically used in a wafer stepping machine for photolithography and also for applications in other areas, for instance precision machine tools. Such devices typically use some sort of electro-magnetic actuator to drive the stage.
These actuators are various types of sophisticated motors which provide linear motion along an axis. One type of motor commonly used in such devices is a voice coil motor which provides bidirectional movement along an axis using a principle similar to that of a loud speaker. (It is to be appreciated that such fine stages only allow a very small amount of linear motion, as described above.)
Voice coil motors provide precision movement, but disadvantageously consume large amounts of electric current and hence produce large amount of waste heat. This is undesirable because the heat adversely affects the precision of the metrology (position measurement) which is required in such systems. Such systems often use laser interferometry to measure the actual position of the stage, and the heat interferes with the interferometry accuracy. Hence, Boone et al. U.S. Pat. No. 5,227,948 discloses a stage moved by two opposing electro-magnetic actuators, each of which is attractive. One such actuator is provided on each side of the stage to pull the stage, either to the left or the right, along the axis. This avoids the need for a voice coil motor. Such push-pull actuator system devices however have relatively complex control characteristics since both actuators must be controlled simultaneously to achieve the desired positioning precision. In such systems, complexity of control is a significant drawback. This is especially the case since in the typical system motion is desired not just along a single axis but actually typically along three axis which are orthogonal to one another, thus providing six degrees of freedom.